Fluid actuated chucks, both pneumatic and hydraulic, are known in the art. U.S. Pat. Nos. 3,770,287, 4,114,909, 4,139,207, 4,147,312, 4,200,301, 4,229,014, 4,317,577, 4,387,906, 4,697,966, 4,771,963, 4,793,053, 4,890,541, 4,946,178, 4,953,877, 4,979,853, 5,030,048, 5,110,146, 5,125,776, 5,127,780, 5,141,370, 5,174,585, 5,237,895, 5,249,815 and 5,259,630 are believed to be representative of the prior art. In general, fluid actuated chucks include a piston which axially drives a centrally located wedge which engages chuck jaws. The axial movement of the wedge causes the jaws to move radially inwards or outwards.
One of the problems of prior art fluid actuated chucks is that the jaws are generally not sealed from possible contamination from chips and machining fluids. In certain machining operations, such as grinding, chips can degrade the performance of the chuck.
In general, the inner sliding portion of the jaws of prior art fluid actuated chucks has a T-shaped cross section. The plurality of tolerances in machining the sliding portion of the jaws and the corresponding channels in the chuck limits the accuracy and repeatability of the chuck usually to no better than one micron.
It is believed that the same plurality of tolerances is the cause of another problem associated with prior art chucks. The problem is that during operation at relatively high rotational speeds, the chuck jaws sometimes have a tendency to "self-open", that is, to loosen their grip on the workpiece.
In a perfectly manufactured chuck, the chuck jaws lie along axes which are coplanar. In actuality, due to the above mentioned tolerances, each chuck jaw does not lie exactly on its corresponding ideal axis. Rather, each end of each chuck jaw lies out of the ideal common plane of the ideal axes. During operation at relatively high rotational speeds, it is believed that relatively large centrifugal forces develop which cause the ends of the chuck jaws to become further displaced from the ideal common plane, thereby causing loosening of the workpiece.